Jointless coded track circuits for railroad signal systems

ABSTRACT

Track currents are supplied to the rails of adjoining sections at each location through a track transformer whose secondary serves as a low impedance cross bond to divide the track into sections without insulated joints and to confine track circuit energy to a given zone. A track transmitter develops sufficient energy for rail currents with the selected different frequencies established by a separate oscillator unit for each adjoining section. A tuned track receiver for each section is independently coupled by pickup coils to the corresponding section rails in the vicinity of the cross bond to receive incoming track current of a different frequency from the other end. Each track current is coded at a selected rate in accord with corresponding advance traffic conditions. A received track current signal is demodulated by the corresponding receiver and applied through decoding units to a logic network which detects track section occupancy and determines the traffic condition. In accord with the registered traffic or occupancy conditions, the logic network selects from code rates provided by a bank of coding devices for modulating the applied track currents. The selected code rates modulate each oscillator output and thus the track transmitter output. If cab signals are used, a separate oscillator, also controlled by the logic means, supplies coded cab signal energy to an approaching train through the track transmitter.

United States Patent Blazek et a1.

[ Feb. 25, 1975 JOINTLESS CODED TRACK CIRCUITS FOR RAILROAD SIGNALSYSTEMS [75] Inventors: Frank V. Blazek; Thomas C.

Vaughn, both of Allegheny, Pa.

[73] Assignee: Westinghouse Air Brake Company, Wilmerding, Pa.

[22] Filed: Sept. 17, 1973 [21] App]. No.: 398,250

Related U.S. Application Data [63] Continuation-in-part 6r Ser. No.276,066, July 28,

1972, abandoned.

[52] U.S. Cl 246/34 CT, 246/63 C, 246/37 [51] Int. Cl 136113/10 [58]Field of Search 246/34 R, 34 CT, 36, 37, 246/63 C [56] Reierences CitedUNITED STATES PATENTS 3,328,581 6/1967 Staples 246/34 CT 3,524,0548/1970 Smith 246/37 3,666,217 5/1972 Sibley 246/36 3,794,833 2/1974Blazek et al. 246/63 C Primary Examiner-M. Henson Wood, Jr. AssistantExaminer-Reinhard J. Eisenzopf Attorney, Agent, or Firm- A. G.Williamson, Jr.; R.

W. Mclntire, Jr.

[57] ABSTRACT Track currents are supplied to the rails of adjoiningsections at each location through a track transformer whose secondaryserves as a low impedance cross bond to divide the track into sectionswithout insulated joints and to confine track circuit energy to a givenzone. A track transmitter develops sufficient energy for rail currentswith the selected different frequencies established by a separateoscillator unit for each adjoining section. A tuned track receiver foreach section is independently coupled by pickup coils to thecorresponding section rails in the vicinity of the cross bond to receiveincoming track current of a different frequency from the other end. Eachtrack current is coded at a selected rate in accord with correspondingadvance traffic conditions. A received track current signal isdemodulated by the corresponding receiver and applied through decodingunits to a logic network which detects track section occupancy anddetermines the traffic condition. ln accord with the registered trafficor occupancy conditions, the logic network selects from code ratesprovided by a bank of coding devices for modulating the applied trackcurrents. The selected code rates modulate each oscillator output andthus the track transmitter output. If cab signals are used, a separateoscillator, also controlled by the logic means, supplies coded cabsignal energy to an approaching train through the track transmitter.

12 Claims, 2 Drawing Figures 11 T 12 3d 14 6T g 5 1 15 l 1 l 1 F3 Fl F3TRACK F4 F2 F4 RECEIVER rnmsmman aacnvcn Fl CABSIGNAL F2 OSCILLATOROSCILLATDR OSCILLATOR oecoozas DEOODERS LOGIC NETWORK WAYSIDE SlGNALSCODERS JOINTLESS CODED TRACK CIRCUITS FOR RAILROAD SIGNAL SYSTEMSBACKGROUND OF THE INVENTION This application is a continuation-in-partof our copending application Ser. No. 276,066, filed July 28, 1972, andnow abandoned. Our invention pertains to jointless track circuits forrailroad signal systems. More particularly, this invention relates tocoded alternating current track circuits without insulated joints fortransmitting a different frequency in each direction in each tracksection to simultaneously detect train occupancy, transmit signalcommands, and establish traffic direction in a signalling system.

Due to rising labor costs, a major disadvantage in prior art signalsystems is the need for insulated rail joints to electrically separatethe adjoining track sections. Another disadvantage in some types ofsignal systems is the requirement for line wires extending along therailroad right-of-way between the various signal locations in order toprovide additional signal controls. In addition to the initialinstallation costs for such elements, the maintenance costs of each itemafter construction are especially burdensome under present economicconditions. Although track circuits without insulated joints areavailable, generally they are too short in length to be practical formain line railroad signal systems. In order to use jointless type trackcircuits in signal systems for long stretches of main line, it isnecessary from an economic standpoint for the track circuits to be of areasonable-minimum length, at least on the order of 5,000 feet. Such asystem using jointless track circuits desirably also requires no waysideline wires between locations except possibly one pair for the supply ofcentral station power to the various locations, unless primary batteryis used, for the operation of apparatus and track circuits. A systemincluding these characteristics is of particular advantage and desirablefor a signaling system for stretches of track between station locations,especially where welded rail with very few joints is being used.

Accordingly, an object of our invention is an arrangement of long,jointless track circuits for railroad signaling systems.

Another object of the invention is a railroad signaling system usingcoded jointless track circuits for train detection and signal commands.

Still another object of our invention is an arrangement of jointlesstrack circuits for a stretch of railroad track to detect the presence oftrains, to establish the direction of traffic, and to transmit signalcommands to wayside and/or cab signal apparatus.

It is also an object of our invention to provide a railroad signalsystem using jointless, coded alternating current track circuits, eachcircuit transmitting a different frequency in each direction through thecircuit length to detect the presence of trains therein, to determinethe direction of approaching trains, and to transmit speed commands tothe trains or to wayside appara- 118.

A further object of the invention is an arrangement of jointless, codedalternating current track circuits which provide an either-directionsignaling system for a stretch of railroad trackwithout line wiresbetween the signal or block locations.

Another object of our invention is a railroad signaling system usingalternating current track circuits of relatively low frequency, withoutinsulated joints between track sections, for train detection and codedenergy to transmit a plurality of signal commands without the use ofwayside line wires.

Further, it is an object of our invention to provide block or tracksection definition in a signaling system without insulated joints byconnecting a low impedance bond across the rails to confine trackcircuit energy to a given block or section in which it is identified bypickup and receiver devices having a specific association therewith.

Other objects, advantages, and features of our invention will becomeapparent from the following description when taken with the accompanyingdrawings and appended claims.

SUMMARY OF THE INVENTION In practicing our invention, the stretch ofrailroad track to be signaled is divided into track sections by lowimpedance cross bonds connected between the rails. However, theadjoining section rails are not insulated from each other by insulatedjoints. Each cross bond is actually one winding of a track transformercoupling a track transmitter device to the rails. The track transmitterunit, principally a power amplifier, is connected to the other i windingof the track transformer and supplies an alternating current of selectedfrequency to the rails of the sections in each direction from the bondlocation. Each track transmitter may thus be a standard unit with theselection of the track frequencies accomplished through the use ofseparate oscillator units, each generating a preselected frequency. Twosuch oscillators are used at each bond location between adjoiningsections. The oscillator output is coded or modulated at a selected ratein accordance with the traffic conditions in advance in the directionfor which the oscillator is assigned. The modulated signal is then applied to the track transmitter which, in turn, provides coded trackcurrent of each frequency.

Two track receiver units, one for each direction from the cross bond,are provided at each location, each tuned to a specific and differentfrequency, differing also from those of the associated oscillators. Eachtrack receiver is coupled to the rails by a pair of energy pickup coilsplaced adjacent to the rails of the corresponding track section in thevicinity of the associated cross bond. These receivers are tuned torespond only to a preselected frequency which is received from anassociated track transmitter at the other end of the track section towhich the receiver is assigned. Each receiver demodulates the codedalternating current received to produce a code rate output which isapplied to decoding units to determine which signal command or trafficcondition indication has been received. Through a logic network, thereceived code rate is effective to select a particular code transmitterwhich controls the oscillator supplying the track frequency for theother track circuit at that location.

In this manner, coded alternating current of different selectedfrequencies is supplied at each end of a track section extending betweenadjacent cross bond locations. Each track section also has two trackreceivers, one at each end, each tuned to receive only alternatingcurrent supplied from the transmitter at the opposite end of the samesection. Each receiver demodulates the received track current andoutputs a code rate or modulating frequency signal. This outputactivates or is passed by the correspondingly tuned decoding unit,

each decoding unitof the associated bank detectingreceiver detects theoccupancy of the corresponding section by a train. To provide for thetransmission of cab signal energy through the rails to the train by thesame track transmitter unit, a single frequency is pro 'vided by a cabsignal oscillator at each location. The

code rate is determined by the traffic conditions in advance, dependingupon the direction of the train approaching throughone of the tracksections.

DETAILED DESCRIPTION OF THE ILLUSTRATEDY EMBODIMENT In describing ingreater detail the apparatus embodying our invention, reference will bemade to the accompanying drawings in which:

FIG. 1 is a schematic block diagram illustrating the apparatus atonelocationin a track circuit arrangementembodying our invention.

FIG. 2 is a diagrammatic illustration of the specific details of a logiccircuit network usable in thearrangement of FIG. 1.

In each of the drawing figures, similar reference characters designatesimilar apparatus. At each location, a source of direct current energyis provided for operation of the apparatus. Since the use of any one ofseveral types of such energy, sources is conventional, a specific sourceis not shown. .Howevenconnections to the 7 positive and negativeterminalsof this source are desig: l

of a portion of railroad track which is part of a stretch of trackextending beyond the drawing figure ineach direction, for example,extending betweentwo station locations. Connected across the rails atthe center of the drawing is one winding of a track transformer 11. Thiswinding is of a relatively low impedance at the alternating currentfrequencies used in the system. It establishes a dividing point betweentrack sections ST and (ST shown, respectively, to the left and right ofthe winding or cross bond connectionbetween the rails. It is emphasized,however, that each rail is electrically continuous from one section tothe other, there being no insulated joints associated with the crossbond connection. Simialr cross bonds or transformer, windings areconnected across the rails at the left and the right of the drawing atthe opposite ends of sectionsST and 6T, respectively. One form of such atrack transformer providing cross bond connections is shown in LettersPatent of'the United States No. 3,328,581, issued June 27, 1967, toCrawford E. Staples, for a Rapid Transit SpeedControl System. Crossbonds usable in our arrangement, depending on the specificcircumstances,

are shown in the various parts of FIG; 16 of this patent. The specificshowing in FIG. 16c illustrates the use of current frequencies and thesame at each location in tional block, which supplies sufficient energyfor the track current of the track circuits of both sections 5T.and6'l". Similar transmitters, not actually shown; will 1 be connectedacross the track transformers at the other end of each tracksection. Thefrequency of the alternating. current used' in the track circuits isestablished by a separate frequency determining means at each lo?cation, shown as a plurality of oscillator units. In other words, eachtrack transmitter isa standard item for the system, is principally apower amplifier for developing sufficientenergylevel for thetraclccurrents, and is de- 7 pendent upon external means to establishfrequency. At this center location in the drawing, associated with thetrack transmitter, are the Pl and F 2 oscillators and a cab signaloscillator. Each of the oscillators for track circuit current, that is,the F1 and F2 oscillators, generates a different preselected frequencyselected from a pluralityof frequencies F1, F2, F3, etc., designated forthe track current. The cab signal oscillator generates a frequencydifferent from any of the plurality of track order tosimplify thetrain-carried cab signal apparatus when used. This last oscillator, ofcourse, is omitted if the trains are not equipped with cab signalapparatus.

Returning to the. other oscillators, to avoid interferencethefrequencies F1. and F2 here used will not be repeatedwithin two or threetrack'sections in succession either wayfrornthe location shown. Thisrepetition within some prefixed distance is possible since the crossbonds are designated toshunt nearly all the track current supplied fromtheother end ofthat section; I

The track currents of these two frequencies generated atthis centerlocation areobviously transmittedin both directions from tracktransformer 11 but, as will become apparent, frequency F1 is effectiveonly in section ST and frequency F2 is effective only in section 6T.This is determined by the tuned receiver units. For.

example, frequencyF3 is supplied at the other end of section 5T, asindicated by the F3 input arrow, and although transmitted in bothdirections, will be received only at the illustrated center location bythe similarly tuned receiver unit, to be described shortly. Likewisetrack current of frequency F4 is supplied to the rails at the other endof section 6T and is receivedonly'by the correspondingly tuned receivershown at the center. lo-

' cation. Each adjacent track circuit in either direction a center tapfrom the cross bond connection if the stretch of railroad iselectrified. Another cross bond. 7

suitable for use in our arrangement is shown inLetters 11 is a tracktransmitter device, shownby a conven- Q will be supplied with .energyofa different pair of track current frequencies.

Each. location is provided with two receiver units,

each tuned to a different frequency, as illustrated at the centerlocation in FIG. 1 byconventionalblocks since such units may be of anyconventional design. One track receiver is coupled to the rails on eachside of the crossbond connection by a pair of energy pickup coils placedadjacent to the rails. Forexample, the F3 receiver is coupled to therails by pickup coils 1.2 and 13,

while receiver F4 is coupled by coils 14 and 15. In each case one. ofthe pair of coilsis pl'acedadjacent to each rail. Energy is induced ineach coil by the track current.

andis similar in frequency and character. Each pair of coils isconnected so that the energy induced is totaled and supplied to theassociatedtrack receiver.

Each receiver unit is tuned to respond only to track current of thedesignated frequency which, incidentally,is that transmittedfrorn theother end of the asso-- ciated sectiomThus, the receiver associated withsection 5T at the illustrated location is tuned to frequency F3 which istransmitted from the left end of the section. Correspondingly, areceiver coupled to the rails at the left end of section 5T would betuned to frequency Fl which is the frequency selected for one currenttransmitted from the illustrated location. Correspondingly, for section6T, the illustrated receiver is tuned to frequency F4 while the receiverat the right end would be tuned to frequency F2. Each receiver is thusnonresponsive to track current of any frequency transmitted from thesame location or to the second frequency current transmitted in multipleat the far end of the associated section. An output from each receiver,when energized, is applied to a bank of decoding units as will beshortly discussed. It is to be noted that the connections for thecouplings of the receiver and track transmitter units to the rails areshown in conventional two-wire symbols, illustrating complete circuits.However, the symbolic connections from the receiver and transmitterunits to the remaining apparatus at the illustrated location are in theform of single line flow channels indicating the flow of energy and/orsignal indications between the various units. Since all units are shownby conventional blocks and any one of various types of apparatus may beused depending upon the specific system, these flow line connectionsrepresent all associated connections and coupling links that arenecessary between the illustrated apparatus.

in order to avoid the use of wayside line wires between locations fordetermining advance traffic conditions and still provide multiple signalindications for controlling train movement, the track current suppliedto the rails is coded or modulated at a very low rate or frequency, ascompared with the alternating current frequency selected for the trackcurrent which carries the code signal. For example, the track currentfrequencies may be selected in the range of 150-600 Hz although higheror lower frequencies in the audio range are also usable. To aconsiderable degree, the desired length of the track circuit determinesthe range of track current frequencies. The code rate or modulatingfrequency for pulsing or coding the track current for additional signalindications is then, by way of example, selected in the range of 1.25 to8.0 Hz. These advance traffic condition signals or code rates aregenerated in any well-known manner. For example, mechanically tuned codetransmitters of the relay types known in the prior art may be used orlow frequency, solid state type oscillators may be employed. For thisreason, the code transmitters or traffic condition signal generators areillustrated in the drawing by conventional blocks only as a bank of fivecoders positioned in the lower center. It should be understood that anytype known in the art or designed for the system may be used. Using fiveseparate code rates, six wayside signal indications, including the stopindication designated by the absence of any code, will be available fortrain contro Thus, the use of five coders is by way of illustration onlyand the actual number will be determined by the number of signalindications and other special track current carried indicationsrequired. As will be understood by those familiar with railwaysignaling, even though five code rates may be utilized in a codedsystem, the generation of all code rates may not be required at eachsignal or between-section location. Nor will every code rate necessarilybe received at each such location.

The pertinent code rate, in accordance with advance traffic conditionsand other speed restrictions, is selected by the logic network unit,which will be shortly described, and separately applied to oscillatorsFl and F2. The selection for each direction is separate and distinct andneed not necessarily result in the same code rate applied to eachoscillator. This code selection results in a modulated output from theoscillator so that, in turn, the track transmitter output to the railsis a coded or pulsed track current at the established frequency andselected code rate. A selected code rate from the coders is also appliedby the logic unit to the cab signal oscillator. A similar result isobtained in which cab signal energy at the predetermined frequency ofthe cab signal oscillator and the selected code rate is applied throughthe track transmitter to the track. The code rate will normally be thesame as that otherwise applied to one or the other of the track circuitoscillators as determined by the direction of approach of the detectedtrain.

The logic network unit, shown by conventional block, is the heart orbrain of the circuitry at each location. This unit receives the outputfrom each decoder bank and accordingly selects the code rate of thetrack current supplied to the corresponding approach, i.e., other,section. in other words, the code rate modulating the track currentreceived from section 5T, as detected by the bank of decoders associatedwith the F3 receiver, determines or selects, through the logic network,the code rate modulating the track current supplied to the approachsection 6T. Conversely, the code rate received through section 6Tdetermines the code rate for the track current applied to section ST.The logic network also, from the received code rates, determines thesignal indications to be displayed by any associated wayside signals.Each receiver and the logic unit may also jointly function without thedecoders, as an occupancy detection means which detects the absence orpresence of a train in the corresponding track section as track currentis or is not, respectively, received by that receiver. lf approachcontrol is used, the logic unit actuates the cab signal oscillator whentrack occupancy is determined to supply cab signal current at the propercode rate from the transmitter into the rails and thus to the apparatuson the approaching train.

Since the actual specific circuitry will depend upon the complexity ofthe signaling system, the logic network in FIG. 1 is shown as aconventional block. Such apparatus and circuitry may be of the relaytype or may be a solid state arrangement. Examples of the relay typefrom the prior art may be found in Letters Patent of the United StatesNo. 2,248,321, issued July 8, 1941, to Earl M. Allen, or No. 2,751,491,issued June 19, l956, to Thomas W. Tizzard. A form of solid statecircuitry, which can be adapted to these logic circuitry requirements,is disclosed in Letters Patent of the United States No. 3,500,388,issued Mar. 10, 1970, to Donald B. Marsh and Walter W. Sanville. Aspecific relay logic network is shown in FIG. 2 and will be discussedshortly.

Each of the bank of five decoder units shown associated with eachreceiver F3 and F4 may be LC tuned circuits or may be active filterunits. In either case, each is designed to pass only a preselected oneof the code rates in use in the signaling system. Thus, the singleoutput flow line from each bank of decoders to the logic unit isactually a multiple path for denoting the code rate selection passed. Atany time, the track current received by the corresponding receiverwillbe modulated by only a single code rate so that only a singledecoding unit will be active to pass a signal to the logic unit todetermine the signal indication to be dis- 1 scribed. Obviously, when notrains are in thetrack.

shown, both banks of decoding units will have one unit active to providean output into the logic network as track current is received from bothdirections by the F3 and F4 receivers. V V 7 Referring now to FIG. 2, aspecific logic circuit network which may be used, in the arrangement ofFIG, 1 is illustrated. Across the top ofthisdrawing, the same stretch oftrack is shown by, a single line representation with portions ofsections of ST and 6T illustrated. The transformer 11, FIG, 1, with itscross bond connection between the rails is located at the pointwhere'signals W5 and E6 are opposite the stretch of track. However,

in this drawing, the track transformer and cross bond connection are notspecifically shown. The wayside signals shown by conventional symbolcontrol the. movetion is provided with code raterepeater relays suchasrelay STCP associated with unit 21, Relay STCP is energized by a simplecircuit including, in multiple, front contactsb of relays STCl and 5TC2and thus is enerment of trains into the correspondingly numbered track Isections, e.g., signal W5 controls movement into section 5T. At theupper left and right of this drawing are conventional blocks 21 and 22which, respectively, designate the track receiver and decoding unitcombinations associated with track sections ST and 6T. The trackcouplings for the receiver units are schematically shown by flow linesas originating at the wayside signal location and are the same as shownin FIG. 1. Although the details are not specifically shown, when thetrack a current energy is coupled to the receiver unit and thus to thedecoders, operating energy from terminals B and N of the direct currentsource is supplied to one of the decoding relays TC associated with theunit selected in accordance with the code rate modulating the receivedtrack current, For example, if the F3 frequency track 7 current flowingthrough section ST and thus coupled to block 21 is modulated by trackcode 1, relay STCl will be energized. Only three code rates 1, 2, and 5are assumed. as being received by either of the track receiver units atthis location although all five code rates are utilized in the totalsystem of which this location is a part. Further, only four code ratesare generated andtransmitted from the illustrated location since thisissufficient to illustrate the concept of operation ofthe system of ourinvention.

Trackoccupancy for the illustrated sections is recorded by the trackrepeater relays ST? and 6TP, re-r spectively. it will be noted, forexample, that relay STP is energized by a circuit which includes inmultiple front contacts a of each of the decoding relays STCl,

5TC2, and STCS associated with receiver-decoder block 21. in the at-restcondition assumed or illustrated in FIG. 2, that is, with no trainsinthe stretch, relay gized and picks up when either of these code rates isreceived at this location through section 5T. If code rates 3 and werealso received at suitable times by receiver and decoder unit'2l, relaySTCP, or a similar auxiliary.

relay, could be used to detect the reception of these codes also. Itwill be noted thatthe track repeater and code repeater relays are eachsnubbed by'a series,

capacitor-resistor arrangement connected in'multiple withthe relaywinding to provide a brief slow release period to bridgepossible opencircuittimes as the decoding relays change positionsduring the change ofcode rate.

The wayside signals are assumed to belof. the search.- light type, forthe convenience of explanation, although this is not critical to thesystem of our invention. The signal mechanisms, including operatingwindings, posi-' tion repeater contacts, and lamps are shown in thelower left and right for signals W5 and E6, respectively.

Such signal mechanisms have a biased winding as designated by thepositiveand negative symbols placed adjacent to the windingrepresentation. It will be understood that, when operating energy isapplied with a ponism. Each contact structure, shown directly below thewinding symbol, has a contact a which closes. in its upper or frontposition when the green aspect is displayed, as shown by the letter Gplaced adjacent the front'contact. The associated contact b is closed inits upper position only when the signal is displaying a yellowindication, as represented by the letter .Y adjacent the uppercontact.Contact a closes in its lower or back position when the signal displayseither a red or yellow indication. and contact b closes in its backposition when the signal is displaying either'a red or a green indication. Since an at-rest condition for the apparatus is assumed, eachsignal is illustrated as displaying a green aspect and contacts a and bof each signal are shown in 5TC2 is energized by the decoders and thusrelay STP v,

the position which they occupy under these conditions. Each signal lampis approached energized and iscontrolled by aback contact of theopposite track repeater relay. For specific example,'when relay 6T?releases on the approach of a train in section 6T, it completes thecircuitover its back contact a for energizing the lamp of signal W5.Correspondingly, the circuit for the lamp of signal E6 includes backcontact a of relay 5TP. The signal lamps are shown as energized by thelocal source of direct current energy but ifdesired,a low voltagealternating current supply may be provided for the signal lamps. V V

The position of each signal is repeated by a pair of position repeatingrelays which are controlled over circuits controlled by the contactstructureof the signal. For example, in the conditions assumed, adistant repeater relay WSDP is energized by a simple circuit betweenterminals B and N which includes front or green contact a of signal W5and the relay winding. The corresponding distant repeater relay E6DP issimilarly energized over front contact a of signal E6. A home anddistant repeater relay is associated with each signal and is energizedover either of two multiple circuits. For example, relay WSHDP isenergized, under the condition shown, by a simple circuit includingfront contact a of relay WSDP. Alternately, relay WSHDP may be energizedwhen signal W5 is displaying a yellow indication over a circuitincluding back contact a and front contact b of the signal contactstructure of signal W5. A similar circuit network controls relay E6HDP.

The center block in FIG. 2, designated 23, represents the tracktransmitter, the track current oscillators, and the coders of FIG. 1 andis shown in this simplified manner for convenience of illustration.Across the bottom of block 23 are two sets of terminals associatedrespectively with track sections 5T andd 6T, as indicated by theoverlined symbol. These terminals are used in connection with theselection of the code rate to be applied to the corresponding tracksection. The terminal C is a common terminal from which a circuit mustbe completed to one of the numbered terminals to select the desired coderate. The numbered terminals 1, 2, 3, and 5 represent thecorrespondingly numbered code rate and provide a selection for thetransmission of such a code rate modulated on the track current offrequencies F1 or F2, respectively.

The selection of code rates involves, in addition to track conditions,the directional stick relays ES and WS, the first being for eastbounddirection and the latter for the westbound direction. conventionally,these directions designate trains moving to the right and, conversely,to the left through the stretch of track shown. These directional stickrelays are used in order to set up conditions to allow a following trainmovement and are energized when a train moving in the correspondingdirection passes this signal location. For example, relay ES isinitially energized when an eastbound train passes signal E6 by acircuit which includes back contact b of relay 6TP, front contact a ofrelay EGHDP, and back contact b of relay STP. Relay ES has two stickcircuits, the primary including back contact b of relay 6TP and frontcontact a and the winding of relay ES. An alternate stick circuitincludes back contact a of relay E6HDP and front contact a of relay ES.lt is to be further noted that both relays ES and WS have inherent slowrelease characteristics, as designated by the downward pointing arrowdrawn through the movable portion of each contact of these relays. Suchrelays, when deenergized, retain front contacts closed for apredetermined slow release period of time. This characterisitc isutilized here to allow the relays to remain picked up during thetransfer between pickup circuits and stick circuits under certainconditions. It is also to be noted that the home and distant repeaterrelays HDP for each signal also include inherent slow releasecharacteristics.

In considering the circuits for the logic network for selecting the coderate to be applied to the track section, it will be sufficient todescribe only those associated with track section 5T as similar circuitarrangements for 6T will become obvious with an understanding of the STcircuits. A first and simple circuit for selecting code rate 5, whensuch is appropriate, extending between terminals C and of the ST side oftrack transmitter block 23, includes front contact b of relay WS. Analternate circuit for selecting code 5 extends between the same twoterminals over back contact b of 5 relay WS, back contact b of relayE6HDP, and back contact 0 of relay ES. One circuit for selecting coderate 1 for section 5T extending between terminals 1 and C of thisportion of block 23 includes back contact d of relay 6TP, front contactb or relay E6l-IDP, and back contact b of relay WS. Another, more normalcircuit for selection of code rate 1 includes front contact c of relayES, back contact b of relay ES, back contact bof relay E6HDP, and backcontact b of relay WS. This latter circuit is used to select code rate 1when a train is receding from the signal location through section 6T.

When a train receding in the eastbound direction from the signallocation has cleared section 6T, the circuit for the selection of coderate 2 for application to track section 5T extends between terminals 2and C of that portion of block 23 over back contact b of relay E6DP,front contact d of relay 6TP, front contact b of relay E6HDP, and backcontact b of relay WS. Under conditions when no train is in the stretchof track and traffic direction is not established, code rate 3 isnormally transmitted in both directions from the signal location shown.A typical circuit for section 5T extends from terminal 3 of that part ofblock 23 over front contact b of relay E6DP, front contact a of relay6TP, front contact b of relay E6HDP, and back contact b of relay WS toterminal C. This is the circuit which is closed under the at-restcondition illustrated in FIG. 2.

We shall now describe the operation of the apparatus when an eastboundtrain moves through section ST and 6T. From FIG. 1, track current atfrequencies F1 and F2 is supplied through track transformer 11 to therails and, with no train in either section, flows through the rails tothe receivers at the opposite ends of sections ST and 6T. The bondsacross the rails at the far end of these sections, as described, have arelatively low impedence at the alternating current frequencies used inthe system so that little, if any, of the track currents of frequenciesF1 and F2 flows in the rails beyond the other ends of sections ST and6T. At the same time, track currents of frequencies F3 and F4 arereceived at the location shown through, respectively, the rails ofsections ST and 6T from the track transmitters at the other end of eachof these sections. Once again, the cross bond winding of transformer 11is of relatively low impedence so that practically all of these currentsflow through this winding and do not appear in any substantial amount inthe other section. This flow of current in sections ST and 6T, throughpickup coils 12, 13 and 14, 15, activates receivers F3 and F4 which aretuned to these frequencies. The energy received is demodulated andapplied to the associated bank of decoders which determine which coderate has been received and pass a corresponding indication to the logicnetwork. Normally, with no train in either section, the cab signaloscillator is not activated by the logic unit which determines that notrain is approaching its location and that there is thus no need for thecab signal current.

We shall assume initially that traffic conditions are still such thatcode rate 2 is received through the track current flowing in bothsections at the location shown in FIG. 2. Under these conditions, eachsignal W5 and E6 displays a green or clear indication, since theassociated TCP and TC2 relays are picked up to apply a reverse polarityto each signal winding. Under such normal conditions, the track currenttransmitted in each directionvis modulated by code rate 3. In otherwords, the track currents of frequencies F l and F2 transmitted fromthis location are modulated by code rate 3 for transmission of signalinformation to adjacent locations. This operation is controlled orexists since both track repeater relays are energized and picked up andboth signal repeater relays for each wayside signal are likewise pickedup. Thus for both sides of track transmitter block 23, a circuit isclosed between terminals C and 3 to actuate transmission of that coderate. As will become apparent later, under this at-rest condition bothdirectional stickrelays are in their released positions.

Some time prior to the assumed eastbound train entering track section5T, the code rate modulating the track current of frequency F3 receivedthrough the rails of section 5T changes to code rate 1. This causesrelay 5TC2 to release and energizes STCI. Although relay STCP remainsenergized and retains its front contacts a and b closed, the shift ofcontacts c and d of relay 5TC2 from front to back changes the polarityon the winding of signal W5 and this signal, now being energized bynormal polarity, shifts to a yellow or approach indication. Since thecorresponding changein the contacts of signal W5 causes relay WSDP torelease, but holds relay WSHDP, the circuit for selecting a code ratefor track current of frequency F2 being transmitted through section 6Tnow shifts from terminal 3 to terminal 2, being transferred by contact bof relay WSDP. In an alternate form of operation with a trafficdirection setup, when a train enters or traffic is established throughthe stretch of track including sections ST and 6T, the track codemodulating all track currents flowing in the eastbound direction isshifted to code rate 5. Under these conditions, at the location in FIG.2, relay 5TC2 releases and relay 5TC5 picks up. Thus relay STCPreleases, interrupting the energy supply to the winding of signal W5,but relay STP remains energized over front contact a of relay STCS.Signal W5 moves to its red position and both signal repeater relaysrelease since there is no circuit for holding either relay energized, Atthe same time, a shift in the logic circuit network occurs to institutethe transmission of code rate 5 modulated onto the F2 track currentflowing through section 6T. The circuit between terminals 5 and C of the6T side of block 23 under these conditions includes back contact b ofrelay ES, back contact b of relay, WSHDP, and back contact c of relayWS. This last described type of operation would occur in CTC or APBterritory and is conventional for such type of signal systems. The shiftto code 5 transmitted in the direction of the train movement occurs atleast when the train enters the overall stretch of track betweenadjacent stations.

When the train moving from left to right through the stretch enterssection 5T, its wheels and axles shunt the flow of track current offrequency F3 so that energy is no longer picked up by coils l2 and 13for application to the F3 receiver. With a deenergized receiver F3, thedecoder, combination of block 21 produces no output and all of theassociated decoding relays TC release to cause the logic circuitry toregister the occupancy of section 5T by this train. Specifically, relaysSTP and 5TCP release to register this track occupancy. The

opening of front contacts a and b of relay STCP, if not previouslyreleased, deenergizes the winding of signal W5 at this time and thesignal moves to its red indication. The closing of back contact e ofrelay 5TP energizes the cab signal oscillator to provide a cab signalcurrent for pick up by the train apparatus if such equipment is in use.The cab signal frequency is applied to the track transmitter block 23and the code rate selected to modulate this current within unit 23depends upon the code rate being received by the F4 receiver from tracksection 6T. Normally the cab signal code rate selected for section ST isthe same as applied to the F1 oscillator prior to the time the trainentered section 5T. However, the rate selected may also be differentdepending upon the train control requirements built into the logiccircuit network. Obviously, from FIG. 1, the cab signal current willalso flow into section 6T where, since no train is present, it has noeffect on any apparatus or operations. i I

As specifically shown in FIG. 2, with the absence of any track currentreceived through section 5T to indicate the presence of a train therein,the logic network selects a code rate for oscillator F2 which reflectsthe fact that the immediate section in advance is occupied by a train.The actual circuit extends from terminal C in the 6T part of block 23over back contact b of relay ES, back contact b of relay WSHDP, and backcontact 0 of relay WS to terminals so that code rate 5 is selected. Ofcourse, as previously described, the overall signal system may be sodesigned that, when the traffic direction is established from left toright, code rate 5 immediately modulates the track currents suppliedfrom left to. right throughout the stretch, including section 6T, thusblocking the opposite direction of train movements.

When the train passes the location shown in the drawings, the trackcurrent supplied through the rails of section 6T from the opposite endis shunted away from pickup coils l4 and 15 which supply energy to theF4 receiver. With receiver F4 thus deenergized, no demodulated code rateis provided to the associated, bank of decoders and relay 6TC2 releases.Since the other decoding relays are already released, relay 6TP and 6TCPalso release and the logic network registers the occupancy of section 6Tby the train. With front contacts a and b of relay 6TCP open, thewinding of sig nal E6 is deenergized and the signal moves to its redindication so that both its front contacts open and the associatedsignal position repeater relays also release. When relay 6TP releases,the circuit is completed for energizing direction relay ES, includingback contact b of relay 6TP, front contact a of relay E6HDP whichremains closed for the release period of this relay, and back contact bof relay STP. The closing of front contact a of relay ES as this relaypicks up completes its stick circuit further including back contact b ofrelay 6TP. The subsequent closing of back contact a of relay E6HDPcompletesa multiple connection to terminal B for this stick circuit. 7

When this train clears section ST and moves on to the right, the F3receiver is again activated since track current now flows from theopposite end of section ST and is picked up by coils l2 and 13 toenergize the receiver. Assuming that at least track code rate 5 isreceived with traffic still established in the eastbound direction,relay 5TC5 is energized and picks up to close its front contact a toalso energize track relay repeater STP. The

logic network thus registers the clearing or the nonoccupancy of sectionST and also selects a code rate for transmission through section 5Treflecting the traffic condition in advance, that is, that section 6T isoccupied with relay ES held in its energized position. The circuit iscompleted for the ST side of transmitter block 23 from terminal C overback contact b of relay WS, back contact b of relay E6HDP, and frontcontact of relay ES to terminal 1. Thus the lowest proceed signal coderate, i.e., code rate 1, is applied to modulate the output of the F1oscillator and produces a track current of frequency Fl coded at thisrate. The F2 oscillator continues to be modulated by code rate 5 sincefront contact b of relay BS is closed to complete circuit betweenterminals 5 and C of this part of transmitter block 23. Thus the coderate applied by the logic network to the F2 oscillator will continue toreflect that traffic direction is established from left to right.However, the application of track current of frequency F2 to the railsof section 6T will have no immediate effect until the section rails areno longer shunted by the train moving through that section. Since thedirection from left to right was previously established as the trainapproached the location, the cab signal oscillator may or may not beactivated while the train is receding through section 6T even thoughback contact 6 of relay 6TP is closed. This is a matter of logic circuitdesign and depends upon the operation desired and the periods duringwhich the cab signal current is to be supplied.

When this train clears section 6T, track current of frequency F4 isagain received through pick up coils l4 and to energize the F4 receiver.Similar to the previous discussion concerning the illustrated locationwhen the train cleared section 5T, the code rate modulating the F4 trackcurrent therefore is the lowest proceed code rate or code rate 1 sinceonly one track section is clear. Relay 6TCl is thus energized at thistime by the track receiver, decoder combination 22 and, with the closingof its front contacts a and b, in turn energizes relays 6TP and 6TCP,respectively. With relay 6TC2 released, the winding of signal E6 isenergized now with normal polarity and signal E6 moves to its yellow orapproach indication. Relay E6HDP is picked up by the closing of frontcontact b of signal E6 while back.

contact a is closed. With relay 6TP picked up to indicate that section6T is clear, the logic circuitry now recognizes that a differentadvanced traffic condition exists and accordingly selects a higher speedcode rate for the F1 oscillator to modulate the track current beingsupplied to section 5T. It will be further noted that, with relays 6TPand E6HDP picked up, their back contacts b and a are open to interruptthe stick circuits for relay ES which shortly releases. The code rate 2is now selected for modulating the track current supplied to section 5T,a circuit existing from terminal C of the ST side of block 23 over backcontact b of relay WS, front contact b of relay E6HDP, front contact dofrelay 6TP, and back contact b of relay E6DP to terminal 2. It will benoted that, with front contact c of relay ES and back contact d of 6TPopen, both circuits for selecting code rate 1 in this side of block 23are interrupted.

When this eastbound train eventually moves on far enough that trackcurrent F4 in section 6T is modulated by code rate 2, unit 22 causesrelay 6TC2 to pick up and relay 6TC1 to release. Relays 6TP and 6TCPremain energized and, with their windings snubbed, do

not open front contacts during the shift in energizing circuits. Theclosing of front contacts 0 and d of relay 6TC2 reverses the polarity ofthe energy applied to signal winding E6 and the signal moves to itsgreen position. Relay E6DP together with relay E6HDP are now energized.The shifting of contact b of relay E6DP from back to front transfers thecode rate selection for section 5T from 2 to 3. If the location at theleft end of section ST is a station location, the reception of code rate3 modulated onto F1 track current may be used, for example, to registerthe stretch of track clear to the next station east and allow a changeof established traffic direction.

Obviously, the operation under the situation of a train moving fromright to left through the track sections shown will be similar but ofopposite sequence to that just described. It is therefore not necessaryto complete a detailed description of this operation as it may bedeveloped by those skilled in the art when considered with the previousdescription and the accompanying drawings.

The arrangement of our invention thus provides a signal system usingcoded a.c. track circuits without insulated joints. The use of differentfrequencies in adja cent track sections maintains the separate systemfunctions of detecting track occupancy by section and transmittingsignal commands section-by-section in accordance with the advancetraffic conditions. The use of coded track current also allows theelimination of wayside line wires between signal locations and yetprovides additional signal indications in accordance wtih the differentcode rates. Maintenance costs of the system are reduced with theelimination of the insulated joints and the wayside line wires. Ourinventive arrangement results, therefore, in an efficient and economicalrailroad signal system.

Although we have herein shown and described but one arrangement ofalternating current track circuits without insulated joints forproviding a railroad signal system, various changes and modificationstherein may be made within the scope of the appended claims withoutdeparting from the spirit and scope of our inven-, tion.

Having thus described the invention, what we claim as new and desire tosecure by the Letters Patent, is:

l. A track circuit arrangement for a stretch of railroad track havingelectrically continuous rails, comprising in combination,

a. a low alternating current impedance cross bond connected across therails at selected locations for dividing said stretch into a series oftrack sections,

b. a track current transmitter means at each location coupled to thecorresponding cross bond and operable for supplying track current to thesection in each direction from the associated bond,

1. each section current having a different preselected frequency anddifferent from the preselected frequencies of a prefixed number ofsuccessive sections in each direction from that particular location,

0. a frequency determining means at each location coupled to theassociated transmitter means for establishing the preselected frequencyof the track current supplied to each section adjoining that location,

d. a separate tuned track receiver coupled to the rails on each side ofthe cross bond connections at each location and responsive only to trackcurrent from the transmitter means at the other end of the correspondingsection forproducing an output signal, e. a single logic network meansat each location conceiving the output signal from the associated trackreceiver and responsive thereto for actuating said logic networkrneansto selectively couple a signal generator to modulate the trackcurrent supplied to arately coding the track current supplied to eachadjoining track section at that location in accordance with the detectedadvance traffic condition in the corresponding direction along saidstretch,

nected to receive the output signal of each track q each track receiverfurther operable for demodureceiver and responsive thereto forregistering a lating the received coded track current to which itnonoccupied indication of the corresponding sec-, is responsive toproduce output signals at a code tion when the signal is present and anoccupied inrate corresponding tothe code rate signal supplied dicationwhen the signal is absent, at the other end of the correspondingsection,

f. a plurality of traffic condition signal generators at r d. eachplurality of decoder units connected for reeach'location, each producinga distinct signal repceiving the code rate outputsignals from theassoresenting a predetermined advance traffic condiciated track receiverand responsive thereto for action along the stretch of track, and matingsaid logic network meansto selectively coug, a plurality of decoderunits, one for each advance plc a code transmitter to code the trackcurrent trafffic condition signal, associated at each location suppliedto the other track section in accordance with each track receiver andcoupled to said logic with the detected advance traffic condition.network means, 5'' A track circuit arrangement as defined in claim 4 h.said signal generators selectively coupled by said which furtherincludes at least at selectedlocations,

logic network means to said frequency determining a. a pair of waysidesignals, each operable for conmeans for separately modulating the trackcurrent trolling train movement in one direction past the supplied toeach adjoining track section in accor-. corresponding location, dancewith the detected advance traffic condition b. each signal controlled bythe associated logic netin the corresponding direction along saidstretch, work for displaying to an approaching train a speed i.eachtrack receiver further operable for demoducommand signalindicationselected in accordance lating the received track current towhich it is:rewith the advance traffic conditions detected for thesponsive to produce a traffic condition output sigcorrespondingdirection. nal corresponding to the modulating signal selected 6. Atrack circuit arrangement as defined in claim 5 at the next adjacentlocation in that direction, which further includes,

j. each plurality of decoder units connected for rea. a pair of energypickup coils associated with each track receiver and positioned one coilin inductive relationship with each rail of the corresponding section inthe vicinity ofsaid cross bond connections, 7 V

the other track section in accordance with the detected advance trafficcondition.

2. A track circuit arrangement as defined in claim! in which saidfrequency determining means at each location includes,

a. a separate oscillator unit for each track section opb.each pair ofcoils connected for supplying to the associated receiver the energyinduced in said coils by'rthe track currents flowing in the sectionrails.

7. A track circuit arrangement as defined in claim 6 in which, at eachlocation,

a. said low impedance cross'bond is the secondary erable to provide anoutput signal of a preselected 4O winding of a track transformer whichalso has a prifrequency different from the signal frequency of marywinding, r the associated oscillator unit and of each oscillator b. saidtrack current transmitter means is connected unit at said prefixednumber of successive locato the primary winding of said tracktransformer to tions, 5 supply the track currents to said rails.

b. said oscillator units connected to apply the output signals to saidtrack transmitter means for jointly establishing the track currentfrequency supplied to the rails at the preselected frequency for eachsection.

8. In a track circuit arrangement for a stretch of railroad track havingelectrically continuous rails, at each of selected locations, thecombination comprising,

a. a track transformer including a first winding and a second lowalternating current impedance winding,

tected occupying either adjoining section and connected to said tracktransmitter means for also supplying a track current to said railshaving said cab. signal frequency.

3. A track circuit arrangement as defined in claim 2 connected acrossthe. rails of said track at the sewhich further includes at eachlocation, lected location to divide said stretch into succesanotheroscillator unit controlled by said logic means sive sections,

to provide a distinct cab signal frequency, the same b. a track currenttransmitter connected to said first at each location, when anapproaching train is dewinding of the associated transformer forsupplying alternating currentenergy tothe rails of each sectionadjoining that location,

. a frequency determining means connected to the associated transmitterfor establishing a different i 4. A track circuit arrangement as definedin claim 3 in which the plurality of traffic condition generators ateach location comprises,

a. a plurality of code transmitters, each producing distinctive coderate, signals representing a differentpredctermined advance trafficcondition along 6 the stretch of track,

b. said code transmitters selectively coupled byv said logic networkmeans to said oscillator units for seprent supplied to the rails ofeachadjoining section, each frequency determining means also havingfrequencies different than those at a preset number of successivelocations in each direction from that location.

d. a separate receiver means coupled to the rails on each side of thesecond transformer winding rail connections at that location and tunedto a predepredetermined frequency for the alternating cur- -17 terminedfrequency for receiving only current of a frequency transmitted from theother end of the corresponding section,

e. a plurality of coding devices, each operable for producing a distinctadvance traffic condition code signal for at times modulating the trackcurrent supplied to the rails of each section,

f. a separate plurality of decoding units coupled to each receivermeans, each decoding unit responsive only to a predetermined one of theplurality of traffic condition code signals,

g. each receiver means responsive to the received code signal carried bycurrent for developing a traffic condition code signal output inaccordance with the modulation by the received track current andconnected for applying the output signal to the associated decodingunits, and

h. a logic network means controlled by both associated pluralities ofdecoding units for detecting the occupancy condition of each adjoiningsection in accordance with the receipt or nonreceipt of a trafficcondition code signal from each section by the corresponding receivermeans,

i. said devices coupled to said frequency determining means by saidlogic network means for selectively modulating each track current with acode signal predetermined in accordance with the advance trafficcondition code signal received from the decoding units associated withthe other track section receiver means.

9. A track circuit arrangement as defined in claim 8 in which eachfrequency determining means comprises,

a. a separate oscillator unit for each track section adjoining thatlocation, operable to provide an output signal frequency of theassociated oscillator unit and of each oscillator unit at said presetnumber of successive locations,

b. said oscillator units at a location controlled by the coding devicesselectively coupled thereto by the associated logic means and connectedto said track transmitter means for jointly establishing thepredetermined frequency and the modulating code signal for the trackcurrent supplied to the rails of each section.

10. A track circuit arrangement as defined in claim 9 which furtherincludes at each location,

another oscillator unit controlledby said logic network means to providea distinct cab signal frequency, the same at each location, when anapproaching train is detected occupying either adjoining section andconnected to said track transmitter means for also supplying a codemodulated track current to said rails having said cab signal frequency.

11. A track circuit arrangement as defined in claim 10 in which,

a. each low impedance second transformer winding terminates themodulated track current supplied from the track transmitter at eachadjacent loca tion, and which each location further includes,

b. a pair of pickup coils inductively coupled to the rails on each sideof the second transformer winding, one coil of each pair positioned ininductive relationship adjacent each rail in the vicinity of theassociated second winding rail connections,

0. each pair of coils connected for supplying to the correspondingreceiver means the energy induced in said coils by the modulated trackcurrents flowing in the section rails,

1. said corresponding receiver means being responsive only to theinduced energy having the frequency to which that receiver is tuned,

2. said corresponding receiver means further responsive to anyinterruption of the rail connections of the second transformer windingat the corresponding adjacent location for actuating the associatedlogic network means to register a track occupied condition for thatsection.

12. A track circuit arrangement as defined in claim 11, in which eachlocation further includes,

a. a pair of wayside signals, each operable for controlling trainmovement in one direction past the corresponding location,

b. each signal controlled by the associated logic network for displayingto an approaching train a speed command signal indication selected inaccordance with the advance traffic conditions detected for thecorresponding direction.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTIONPATENT NO. 3,868,075 DATED February 25, 1975 INV'ENTOR(S) Frank V.Blazek 80 Thomas C. Vaughn It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 17, line 13, erase "code signal carried by" and insert --track--line 15, after "modulation" erase "by" and insert --code signal carriedby-- line 2M, before "devices" insert --coding-- line 35, after "signal"insert --of a predetermined-- same line, after "frequency" insert--different from the signal frequency-- line 40, before "means" insert-'-network-- Signed and Scaled this Fourth Day Of January 1977 [SEAL].Attest:

RUTH C. MASON Arresting Officer C. MARSHQLL DANN Commissioner ufParenrsand Trademarks

1. A track circuit arrangement for a stretch of railroad track havingelectrically continuous rails, comprising in combination, a. a lowalternating current impedance cross bond connected across the rails atselected locations for dividing said stretcH into a series of tracksections, b. a track current transmitter means at each location coupledto the corresponding cross bond and operable for supplying track currentto the section in each direction from the associated bond,
 1. eachsection current having a different preselected frequency and differentfrom the preselected frequencies of a prefixed number of successivesections in each direction from that particular location, c. a frequencydetermining means at each location coupled to the associated transmittermeans for establishing the preselected frequency of the track currentsupplied to each section adjoining that location, d. a separate tunedtrack receiver coupled to the rails on each side of the cross bondconnections at each location and responsive only to track current fromthe transmitter means at the other end of the corresponding section forproducing an output signal, e. a single logic network means at eachlocation connected to receive the output signal of each track receiverand responsive thereto for registering a nonoccupied indication of thecorresponding section when the signal is present and an occupiedindication when the signal is absent, f. a plurality of trafficcondition signal generators at each location, each producing a distinctsignal representing a predetermined advance traffic condition along thestretch of track, and g. a plurality of decoder units, one for eachadvance trafffic condition signal, associated at each location with eachtrack receiver and coupled to said logic network means, h. said signalgenerators selectively coupled by said logic network means to saidfrequency determining means for separately modulating the track currentsupplied to each adjoining track section in accordance with the detectedadvance traffic condition in the corresponding direction along saidstretch, i. each track receiver further operable for demodulating thereceived track current to which it is responsive to produce a trafficcondition output signal corresponding to the modulating signal selectedat the next adjacent location in that direction, j. each plurality ofdecoder units connected for receiving the output signal from theassociated track receiver and responsive thereto for actuating saidlogic network means to selectively couple a signal generator to modulatethe track current supplied to the other track section in accordance withthe detected advance traffic condition.
 2. said corresponding receivermeans further responsive to any interruption of the rail connections ofthe second transformer winding at the corresponding adjacent locationfor actuating the associated logic network means to register a trackoccupied condition for that section.
 2. A track circuit arrangement asdefined in claim 1 in which said frequency determining means at eachlocation includes, a. a separate oscillator unit for each track sectionoperable to provide an output signal of a preselected frequencydifferent from the signal frequency of the associated oscillator unitand of each oscillator unit at said prefixed number of successivelocations, b. said oscillator units connected to apply the outputsignals to said track transmitter means for jointly establishing thetrack current frequency supplied to the rails at the preselectedfrequency for each section.
 3. A track circuit arrangement as defined inclaim 2 which further includes at each location, another oscillator unitcontrolled by said logic means to provide a distinct cab signalfrequency, the same at each location, when an approaching train isdetected occupying either adjoining section and connected to said tracktransmitter means for also supplying a track current to said railshaving said cab signal frequency.
 4. A track circuit arrangement asdefined in claim 3 in which the plurality of traffic conditiongenerators at each location comprises, a. a plurality of codetransmitters, each producing distinctive code rate signals representinga different predetermined advance traffic condition along the stretch oftrack, b. said code transmitters selectively coupled by said logicnetwork means to said oscillator units for separately coding the trackcurrent supplied to each adjoining trAck section at that location inaccordance with the detected advance traffic condition in thecorresponding direction along said stretch, c. each track receiverfurther operable for demodulating the received coded track current towhich it is responsive to produce output signals at a code ratecorresponding to the code rate signal supplied at the other end of thecorresponding section, d. each plurality of decoder units connected forreceiving the code rate output signals from the associated trackreceiver and responsive thereto for actuating said logic network meansto selectively couple a code transmitter to code the track currentsupplied to the other track section in accordance with the detectedadvance traffic condition.
 5. A track circuit arrangement as defined inclaim 4 which further includes at least at selected locations, a. a pairof wayside signals, each operable for controlling train movement in onedirection past the corresponding location, b. each signal controlled bythe associated logic network for displaying to an approaching train aspeed command signal indication selected in accordance with the advancetraffic conditions detected for the corresponding direction.
 6. A trackcircuit arrangement as defined in claim 5 which further includes, a. apair of energy pickup coils associated with each track receiver andpositioned one coil in inductive relationship with each rail of thecorresponding section in the vicinity of said cross bond connections, b.each pair of coils connected for supplying to the associated receiverthe energy induced in said coils by the track currents flowing in thesection rails.
 7. A track circuit arrangement as defined in claim 6 inwhich, at each location, a. said low impedance cross bond is thesecondary winding of a track transformer which also has a primarywinding, b. said track current transmitter means is connected to theprimary winding of said track transformer to supply the track currentsto said rails.
 8. In a track circuit arrangement for a stretch ofrailroad track having electrically continuous rails, at each of selectedlocations, the combination comprising, a. a track transformer includinga first winding and a second low alternating current impedance windingconnected across the rails of said track at the selected location todivide said stretch into successive sections, b. a track currenttransmitter connected to said first winding of the associatedtransformer for supplying alternating current energy to the rails ofeach section adjoining that location, c. a frequency determining meansconnected to the associated transmitter for establishing a differentpredetermined frequency for the alternating current supplied to therails of each adjoining section, each frequency determining means alsohaving frequencies different than those at a preset number of successivelocations in each direction from that location, d. a separate receivermeans coupled to the rails on each side of the second transformerwinding rail connections at that location and tuned to a predeterminedfrequency for receiving only current of a frequency transmitted from theother end of the corresponding section, e. a plurality of codingdevices, each operable for producing a distinct advance trafficcondition code signal for at times modulating the track current suppliedto the rails of each section, f. a separate plurality of decoding unitscoupled to each receiver means, each decoding unit responsive only to apredetermined one of the plurality of traffic condition code signals, g.each receiver means responsive to the received code signal carried bycurrent for developing a traffic condition code signal output inaccordance with the modulation by the received track current andconnected for applying the output signal to the associated decodingunits, and h. a logic network means controlled by both associatedpluralities of decoding units for detecting the ocCupancy condition ofeach adjoining section in accordance with the receipt or nonreceipt of atraffic condition code signal from each section by the correspondingreceiver means, i. said devices coupled to said frequency determiningmeans by said logic network means for selectively modulating each trackcurrent with a code signal predetermined in accordance with the advancetraffic condition code signal received from the decoding unitsassociated with the other track section receiver means.
 9. A trackcircuit arrangement as defined in claim 8 in which each frequencydetermining means comprises, a. a separate oscillator unit for eachtrack section adjoining that location, operable to provide an outputsignal frequency of the associated oscillator unit and of eachoscillator unit at said preset number of successive locations, b. saidoscillator units at a location controlled by the coding devicesselectively coupled thereto by the associated logic means and connectedto said track transmitter means for jointly establishing thepredetermined frequency and the modulating code signal for the trackcurrent supplied to the rails of each section.
 10. A track circuitarrangement as defined in claim 9 which further includes at eachlocation, another oscillator unit controlled by said logic network meansto provide a distinct cab signal frequency, the same at each location,when an approaching train is detected occupying either adjoining sectionand connected to said track transmitter means for also supplying a codemodulated track current to said rails having said cab signal frequency.11. A track circuit arrangement as defined in claim 10 in which, a. eachlow impedance second transformer winding terminates the modulated trackcurrent supplied from the track transmitter at each adjacent location,and which each location further includes, b. a pair of pickup coilsinductively coupled to the rails on each side of the second transformerwinding, one coil of each pair positioned in inductive relationshipadjacent each rail in the vicinity of the associated second winding railconnections, c. each pair of coils connected for supplying to thecorresponding receiver means the energy induced in said coils by themodulated track currents flowing in the section rails,
 12. A trackcircuit arrangement as defined in claim 11, in which each locationfurther includes, a. a pair of wayside signals, each operable forcontrolling train movement in one direction past the correspondinglocation, b. each signal controlled by the associated logic network fordisplaying to an approaching train a speed command signal indicationselected in accordance with the advance traffic conditions detected forthe corresponding direction.